Method and apparatus for detecting a planarized outer layer of a semiconductor wafer with a confocal optical system

ABSTRACT

A method of planarizing a first side of a semiconductor wafer with a polishing system includes the step of polishing the first side of the wafer in order to remove material from the wafer. The method also includes the step of moving a lens of a confocal optical system between a number of lens positions so as to maintain focus on the first side of the wafer during the polishing step. The method further includes the step of determining a rate-of-movement value based on movement of the lens during the moving step. Moreover, the method includes the step of stopping the polishing step if the rate-of-movement value has a predetermined relationship with a movement threshold value. An apparatus for polishing a first side of a semiconductor wafer is also disclosed.

This application is a divisional of U.S. application Ser. No.09/177,335, filed Oct. 22, 1998, now U.S. Pat. No. 6,201,253.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to a method and apparatus fordetecting a planarized outer layer of a semiconductor wafer, and moreparticularly to a method and apparatus for detecting a planarized outerlayer of a semiconductor wafer by monitoring movement of an objectivelens associated with a confocal optical system during polishing of thesemiconductor wafer.

BACKGROUND OF THE INVENTION

Semiconductor integrated circuits are typically fabricated by a layeringprocess in which several layers of material are fabricated on or in asurface of a wafer, or alternatively, on a surface of a previous layer.This fabrication process typically requires subsequent layers to befabricated upon a smooth, planar surface of a previous layer. However,the surface topography of layers may be uneven due to an uneventopography associated with an underlying layer. As a result, a layer mayneed to be polished in order to present a smooth, planar surface for asubsequent processing step. For example, a layer may need to be polishedprior to formation of a conductor layer or pattern on an outer surfaceof the layer.

In general, a semiconductor wafer may be polished to remove hightopography and surface defects such as scratches, roughness, or embeddedparticles of dirt or dust. The polishing process typically isaccomplished with a polishing system that includes top and bottomplatens (e.g. a polishing table and a wafer carrier or holder), betweenwhich the semiconductor wafer is positioned. The platens are movedrelative to each other thereby causing material to be removed from thesurface of the wafer. This polishing process is often referred to asmechanical planarization (MP) and is utilized to improve the quality andreliability of semiconductor devices. The polishing process may alsoinvolve the introduction of a chemical slurry to facilitate higherremoval rates, along with the selective removal of materials fabricatedon the semiconductor wafer. This polishing process is often referred toas chemical mechanical planarization or chemical mechanical polishing(CMP).

In these polishing processes, it is often important to determine when anouter layer or film has been polished to a desired planarity level. Inparticular, it is desirable to know when the outer layer of thesemiconductor wafer has been polished to a planarity level which isacceptable for presentation of the wafer to a subsequent fabricationprocess.

In order to determine when a wafer has been polished to a desiredplanarity level, systems and techniques have heretofore been utilizedwhich polish the wafer down to a predetermined thickness. For example, atypical method employed for determining when the wafer has been polisheddown to a predetermined thickness is to measure the amount of timeneeded to planarize a first wafer to the desired thickness, andthereafter polishing the remaining wafers for a similar amount of time.In practice this method is extremely time consuming since machineoperators must inspect each wafer (e.g. measure the thickness thereof)after polishing. In particular, it is extremely difficult to preciselycontrol the removal rate of material since the removal rate may varyduring the polishing of an individual wafer. Moreover, the removal ratemay be diminished in the process of polishing a number of wafers insequence. Yet further, such methods do not actually measure theplanarity of the outer layer, but rather simply make an assumption thatthe outer layer has been polished to an acceptable planarity level whenthe wafer is polished to the desired thickness.

Another method employed for determining if the wafer has reached thedesired thickness is to impinge a light beam, such as a laser lightbeam, onto the semiconductor wafer in order to determine the thicknessof the wafer. Various techniques have been used to detect when an outerfilm associated with the semiconductor wafer reaches the desiredthickness. For example, the apparatus disclosed in U.S. Pat. No.5,151,584 issued to Ebbing et al directs an incident laser beam onto thesurface of a semi-transparent thin film (e.g. silicon dioxide) of asemiconductor wafer during etching thereof. A first portion of theincident beam is reflected from the top surface of the film, and asecond portion of the incident beam is reflected from the bottom surfaceof the film. Since the film has a finite thickness, the two reflectionswill either constructively or destructively interfere with one another.As the layer is etched, its thickness is changed thereby cyclingintensity of the reflected beam through constructive and destructiveinterference patterns which may be utilized to determine when the waferhas been etched to the desired thickness. Such a technique has a numberof drawbacks associated therewith. For example, such a technique mayonly be utilized after certain steps in the fabrication process. Forexample, such a technique may be useful for measuring thickness of ablank wafer, but has been found to perform unsatisfactorily whenutilized to measure thickness of a patterned wafer. Moreover, similarlyto the manual inspection method discussed above, such a technique doesnot actually measure the planarity of the outer layer, but rather simplymakes an assumption that the outer layer has been polished to anacceptable planarity level when the wafer is etched down to the desiredthickness.

In order to overcome the above-mentioned drawbacks associated with waferthickness-based polishing endpoint techniques, a number of techniqueshave heretofore been utilized in an attempt to measure the actualplanarity of the outer layer of the wafer. For example, a method whichhas heretofore been employed for determining when the wafer has beenpolished to a desired planarity level is to periodically remove thewafer from the polishing system, and thereafter measure the planarity ofthe wafer with an instrument such as an atomic force microscope or aprofilometer. If the wafer has been polished to the desired planaritylevel, the wafer is released to a subsequent fabrication step. However,if the wafer has not been polished to the desired planarity level, thewafer must be placed back into the polishing system for furtherpolishing thereof. It should be appreciated that numerous measurementsmay be required to reach the desired planarity level. Hence, in practicethis method is extremely time consuming since machine operators mustmeasure each wafer (i.e. measure the planarity thereof) a number oftimes during the polishing process.

Thus, a continuing need exists for a method and an apparatus for in situmeasurement of the planarity of the outer layer of a semiconductor waferduring polishing thereof.

SUMMARY OF THE INVENTION

In accordance with a first embodiment of the present invention, there isprovided a method of planarizing a first side of a semiconductor waferwith a polishing system. The method includes the step of polishing thefirst side of the wafer in order to remove material from the wafer. Themethod also includes the step of moving a lens of a confocal opticalsystem between a number of lens positions so as to maintain focus on thefirst side of the wafer during the polishing step. The method furtherincludes the step of determining a rate-of-movement value based onmovement of the lens during the moving step. Moreover, the methodincludes the step of stopping the polishing step if the rate-of-movementvalue has a predetermined relationship with a movement threshold value.

Pursuant to a second embodiment of the present invention, there isprovided a method of planarizing a first side of a semiconductor wafer.The method includes the step of polishing the first side of the wafer inorder to remove material from the wafer. The method also includes thestep of transmitting a first incident light beam from a confocal opticalsystem during a first time period. The first incident light beamimpinges on the first side of the wafer during the polishing step so asto form a first reflected light beam which is reflected from the firstside of the wafer. The method further includes the step of analyzing thefirst reflected light beam so as to determine if the confocal opticalsystem is focused on the first side of the wafer during the first timeperiod. The method yet further includes the step of transmitting asecond incident light beam from the confocal optical system during asecond time period. The second incident light beam impinges on the firstside of the wafer during the polishing step so as to form a secondreflected light beam which is reflected from the first side of thewafer. The method moreover includes the step of analyzing the secondreflected light beam so as to determine if the confocal optical systemis focused on the first side of the wafer during the second time period.Finally, the method includes the step of stopping the polishing step ifthe confocal optical system is focused on the first side of the waferduring both the first time period and the second time period.

Pursuant to a third embodiment of the present invention, there isprovided an apparatus for polishing a first side of a semiconductorwafer. The apparatus includes a polishing system which operates topolish the wafer. The polishing system has a polishing platen whichincludes a polishing surface, and a wafer carrier which is configured toengage the wafer by a second side of the wafer, and apply pressure tothe wafer in order to press the wafer against the polishing surface ofthe polishing platen. The apparatus also includes a confocal opticalsystem having a movable objective lens. The confocal optical system isconfigured to move the objective lens between a number of lens positionsso as to maintain focus on the first side of the wafer during polishingof the wafer. The apparatus further includes a controller electricallycoupled to the confocal optical system. The controller is configured todetermine a rate-of-movement value based on movement of the objectivelens during polishing of the wafer, and terminate operation of thepolishing system so as to cease polishing of the wafer in response todetermination that the rate-of-movement value has a predeterminedrelationship with a movement threshold value.

Pursuant to a fourth embodiment of the present invention, there isprovided an apparatus for polishing a first side of a semiconductorwafer. The apparatus includes a polishing system which operates topolish the wafer. The polishing system has a polishing platen whichincludes a polishing surface. The polishing system also includes a wafercarrier which is configured to engage the wafer by a second side of thewafer and apply pressure to the wafer in order to press the waferagainst the polishing surface of the polishing platen. The apparatusalso includes a confocal optical system positioned such that a firstincident light beam transmitted by the confocal optical system isimpinged upon the first side of the wafer during a first period of timeso as to form a first reflected light beam which is reflected from thefirst side of the wafer. The confocal optical system is also positionedsuch that a second incident light beam transmitted by the confocaloptical system is impinged upon the first side of the wafer during asecond period of time so as to form a second reflected light beam whichis reflected from the first side of the wafer. Yet further, the confocaloptical system is positioned such that the first and second reflectedlight beams are received with the confocal optical system. The apparatusalso includes a controller electrically coupled to the confocal opticalsystem. The controller is configured to analyze the first reflectedlight beam so as to determine if the confocal optical system is focusedon the first side of the wafer during the first time period, analyze thesecond reflected light beam so as to determine if the confocal opticalsystem is focused on the first side of the wafer during the second timeperiod, and terminate operation of the polishing system so as to ceasepolishing of the wafer in response to determination that the confocaloptical system is focused on the first side of the wafer during both thefirst time period and the second time period.

It is an object of the present invention to provide a new and usefulmethod and apparatus for determining when a semiconductor wafer has beenpolished to a desired planarity level.

It is also an object of the present invention to provide an improvedmethod and apparatus for determining when a semiconductor wafer has beenpolished to a desired planarity level.

It is yet further an object of the present invention to provide a methodand apparatus for determining when a semiconductor wafer has beenpolished to a desired planarity level that is less mechanically complexrelative to polishing systems which have heretofore been designed.

It is moreover an object of the present invention to provide a methodand apparatus for determining when a semiconductor wafer has beenpolished to a desired planarity level that is less mechanically complexrelative to polishing systems which have heretofore been designed, yetdetects the planarity level of the semiconductor wafer during polishingthereof.

It is also an object of the present invention to provide a method andapparatus for determining when a semiconductor wafer has been polishedto a desired planarity level which does not require chemical analysis ofthe slurry associated with the polishing system.

The above and other objects, features, and advantages of the presentinvention will become apparent from the following description and theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1F show sectional views of a semiconductor wafer during varioussteps of a fabrication process;

FIG. 2 is a diagrammatic view of a polishing system which incorporatesvarious features of the present invention therein;

FIG. 3 is a top elevational view of the platen assembly of the polishingsystem of FIG. 2;

FIG. 4 is a diagrammatic view of a first embodiment of the confocaloptical system associated with the polishing system of FIG. 2;

FIG. 5 is a view similar to FIG. 4, but showing a second embodiment ofthe confocal optical system; and

FIG. 6 shows a flowchart of a polishing procedure used by the polishingsystem of FIGS. 2.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and will herein be described in detail. Itshould be understood, however, that there is no intent to limit theinvention to the particular forms disclosed, but on the contrary, theintention is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theappended claims.

Referring now to FIGS. 1A-1F, there is shown a semiconductor wafer 10after various steps of a fabrication process of the present invention.In particular, as shown in FIGS. 1A and 1B, the semiconductor wafer 10includes a semiconductor substrate 12, such as silicon. A firstinsulating layer 14 and a first metal layer 16 are deposited orotherwise disposed on the semiconductor substrate 12. More specifically,the fabrication process deposits the first insulating layer 14 on thesemiconductor substrate 12 such that a contact hole 20 is formed in thefirst insulating layer 14 at a location above a transistor portion ofthe semiconductor substrate 12. Moreover, the fabrication processpatterns the first metal layer 16 (e.g. aluminum) over the firstinsulating layer 14 and the contact hole 20. As a result, the firstmetal layer 16 fills the contact hole 20 thereby forming an electricalcontact with the transistor portion of the semiconductor substrate 12.Moreover, the filling of the contact hole 20 forms a pit 22 in theportion of the first metal layer 16 disposed above the contact hole 20.

As shown in FIG. 1C, a second insulating layer 24 is deposited on theouter surface of the first insulating layer 14 and the first metal layer16. The second insulating layer 24 has an uneven surface topography as aresult of the varying topography associated with the first insulatinglayer 14 and a first metal layer 16. The uneven surface topography ofthe second insulating layer 24 may cause accuracy problems infabricating additional layers associated with the semiconductor wafer10. For example, the uneven surface topography may cause accuracyproblems for a lithography process which is utilized to pattern a secondmetal layer 26 (FIG. 1F) on the second insulating layer 24. As shall bediscussed below in more detail, in order to avoid such accuracy problemsassociated with the uneven topography of the second insulating layer 24,a polishing system, such as a polishing system 30 of FIG. 2, polishesthe second insulating layer 24 so as to produce a planar surface 28 (seeFIG. 1D) having a desired planarity level.

As alluded to above, once the semiconductor wafer 10 has been polishedto the desired planarity level, additional layers may be deposited orotherwise fabricated thereon. For example, as shown in FIGS. 1E and 1F,a via hole 36 may be etched through the second insulating layer 24.Thereafter, the second metal layer 26 may be deposited on the secondinsulating layer 24. It should be appreciated that numerous additionallayers may be deposited on the semiconductor wafer 10 in the mannerpreviously described.

Referring now to FIG. 2, there is shown a preferred embodiment of thepolishing system 30 which is used to planarize a front side or surface38 of the semiconductor wafer 10. The polishing system 30 includes aplaten motor or other drive mechanism 40 and a platen assembly 42. Theplaten motor 40 rotates the platen assembly 42 about a center axis 44.The platen motor 40 may rotate the platen assembly 42 in a clockwisedirection (as shown by arrow 46 of FIG. 2) or in the counterclockwisedirection.

The platen assembly 42 includes a polishing platen 48 and a polishingpad 50 mounted on the polishing platen 48. Both the polishing platen 48and the polishing pad 50 are preferably circular and collectively definea polishing surface against which the front side 38 of the semiconductorwafer 10 may be polished. Moreover, the polishing pad 50 is typicallymade of blown polyurethane which protects the polishing platen 48 fromchemical slurry and other chemicals introduced during the polishingprocess.

The polishing system 30 also includes a polishing head assembly 52. Thepolishing head assembly 52 includes a wafer carrier 54, a coolingmechanism 56, a wafer carrier motor or other drive mechanism 58, and awafer carrier displacement mechanism 60. The wafer carrier 54 applies acontrolled, adjustable force in the general direction of arrow 62 inorder to press the front side 38 of the semiconductor wafer 10 intocontact with the polishing pad 50 so as to facilitate polishing of thefront side 38 of the semiconductor wafer 10.

The wafer carrier motor 58 rotates the wafer carrier 54 and thesemiconductor wafer 10 about a center axis 64. The wafer carrier motor58 may rotate the wafer carrier 54 in a clockwise direction (as shown byarrow 66 of FIG. 2) or in the counterclockwise direction. However, thewafer carrier motor 58 preferably rotates the wafer carrier 54 in thesame rotational direction as the platen motor 40 rotates the platenassembly 42 (although the wafer carrier motor 58 may rotate thesemiconductor wafer 10 in the rotational direction opposite therotational direction of the platen assembly 42 as desired).

The wafer carrier 54 also includes mechanisms (not shown) for holdingthe semiconductor wafer 10. For example, the wafer carrier 54 mayinclude a vacuum-type mechanism which generates a vacuum force thatdraws the semiconductor wafer 10 against the wafer carrier 54. Once thesemiconductor wafer 10 is positioned on the wafer carrier 54 and held incontact with the platen assembly 42 for polishing, the vacuum force maybe removed. In such an arrangement, the wafer carrier 54 may be designedwith a friction surface or a carrier pad which engages a back side 70 ofthe semiconductor wafer 10 with a carrier ring (not shown). Such acarrier pad, along with the force being applied in the general directionof arrow 62, creates a frictional force between the wafer carrier 54 andthe semiconductor wafer 10 that effectively holds the semiconductorwafer 10 against the wafer carrier 54 thereby causing the semiconductorwafer 10 to rotate at the same velocity as the wafer carrier 54. Itshould be appreciated that such wafer carriers and carrier pads are ofconventional design and are commercially available.

The cooling mechanism 56 counteracts heat generated during the polishingprocess in order to maintain the wafer carrier 54 at a substantiallyconstant temperature. In particular, the cooling mechanism 56neutralizes the heat generated due to friction and a chemical slurryreacting with the front side 38 of the semiconductor wafer 10. Moreover,it should be appreciated that the polishing system 30 may also includean additional cooling mechanism (not shown) for cooling the componentsof the polishing assembly 42 (e.g. the polishing platen 48) duringpolishing of the semiconductor wafer 10.

The displacement mechanism 60 selectively moves the wafer carrier 54 andhence the semiconductor wafer 10 across the platen assembly 42 in thegeneral direction of arrows 68 and 98. Such movement defines a polishingpath which may be linear, sinusoidal, or a variety of other patterns.The displacement mechanism 60 is also capable of moving thesemiconductor wafer 10 along a polishing path to a location beyond theedge of the polishing pad 50 so that the semiconductor wafer 10“overhangs” the edge. Such an overhanging arrangement permits thesemiconductor wafer 10 to be moved partially on and partially off thepolishing pad 50 to compensate for polishing irregularities caused by arelative velocity differential between the faster moving outer portionsand the slower moving inner portions of the platen assembly 42.

The polishing system 30 also includes a chemical slurry system 72. Theslurry system 72 includes a slurry storage reservoir 74, a slurry flowcontrol mechanism 76, and a slurry conduit 78. The slurry storagereservoir 74 includes one or more containers for storing slurry. Inparticular, the slurry storage reservoir 74 contains a chemical slurrythat includes abrasive material which facilitates polishing of the frontside 38 of the semiconductor wafer 10. Chemical slurries having suchproperties are well known and commercially available.

The slurry flow control mechanism 76 controls the flow of slurry fromthe slurry storage 74, through the slurry conduit 78, and onto thepolishing area atop the platen assembly 42. Hence, the slurry flowcontrol mechanism 76 and the slurry conduit 78 selectively introduce aflow of slurry (as indicated by arrow 80) atop the polishing pad 50.

In order to determine when the polishing system 30 has polished thesemiconductor wafer to the desired planarity level, there is provided anendpoint detection system 150. As shown in FIGS. 2 and 4, the endpointdetection system 150 includes an confocal optical system 152. Theconfocal optical system 152 includes a light source such as a laserlight source 154, a beam splitter 156, an objective lens 158, a lenspositioning device 160, a photodetector 162, and an optics controller164. Preferably, the confocal optical system 152 is embodied as aconfocal laser system. One such confocal laser system which is suitablefor use as the confocal optical system 152 of the present invention isdisclosed in U.S. Pat. No. 4,689,491 issued to Lindow et al, thedisclosure of which is hereby incorporated by reference. Moreover,numerous types of commercially available confocal laser systems may alsobe utilized as the confocal optical system 152 of the present invention.One such commercially available confocal laser system which isparticularly useful as the confocal optical system 152 of the presentinvention is a Model Number 1010 confocal laser system which iscommercially available from KLA-Tencor Corporation of San Jose, Calif.

The confocal optical system 152 is provided to monitor polishing of thesemiconductor wafer 10 in order to determine when the first side 38thereof has been planarized to a desired level (i.e. when the planarsurface 28 has been produced). In particular during polishing of thesemiconductor wafer 10 the confocal optical system generates incidentlaser beams which are directed through an opening 166 defined in theplaten assembly 42 (see FIG. 3) and are impinged on the front side 38 ofthe semiconductor wafer 10. Reflected laser beams are reflected back tothe confocal optical system 152 based on the degree of planarity of thefront side of the semiconductor wafer 10. It should be appreciated that,as shall be discussed below in greater detail, the direction ordirections in which incident laser beams is/are reflected or otherwiseredirected is dependent on the surface topography of the front side 38of the semiconductor wafer 10.

The optics controller 164 analyzes reflected laser beams in order todetermine when the front side 38 of the semiconductor wafer 10 has beenpolished down to a desired planarity level. In particular, duringpolishing of the semiconductor wafer 10, the laser source 154 generatesa laser beam which is directed into the beam splitter 156 so as to bedirected through the objective lens 158, the opening 166 defined in theplaten assembly 42 (see FIG. 3), and thereafter impinged on the frontside 38 of the semiconductor wafer 10. Reflected laser beams arereflected back from a planar feature (if one is present) at the focalplane, through the opening 166, the objective lens 158, the beamsplitter 156, and are thereafter detected by the photodetector 162. Theoutput of the photodetector 162 is indicative of the intensity level ofthe reflected laser beam from the front side 38 of the wafer 10 and istransmitted to the optics controller 164 via a signal line 168. Theoptics controller 164 then adjusts the position of the objective lens158 in order to focus the confocal optical system 152 on the front side38 of the semiconductor wafer 10. In particular, the optics controller164 determines if the intensity level of the reflected laser beam iswithin a predetermined light intensity range. The predetermined lightintensity range is generally indicative of a maximum intensity levelassociated with reflected laser beams from a planar surface located atthe focal plane of the confocal optical system 152. Hence, as usedherein, the confocal optical system 152 is “focused” or “maintainsfocus” on the front side 38 of the semiconductor wafer 10 when theintensity level of reflected laser beams (as reflected from the frontside 38 of the wafer 10) is within the predetermined light intensityrange.

The optics controller 164 provides closed-loop control of the positionof the objective lens 158 in order to maintain focus on the front side38 of the semiconductor wafer 10. In particular, if the intensity levelof a given reflected laser beam is not within the predetermined lightintensity range thereby indicating that the confocal optical system 152is not focused on the front side 38 of the semiconductor wafer 10, theoptics controller 164 communicates with the lens positioning device 160so as to move the objective lens 158 upwardly or downwardly (i.e. in thegeneral directions of arrows 170 or 172 of FIG. 4, respectively) inorder to focus the confocal optical system 152 onto the front side 38 ofthe semiconductor wafer 10. More specifically, the optics controller 164generates an output signal on a signal line 174 thereby causing the lenspositioning device 160 to move the objective lens 158 either upwardly ordownwardly (as required). Thereafter, the optics controller 164communicates with the laser source 154 via a signal line 176 in order togenerate another incident laser beam which is impinged on the front side38 of the semiconductor wafer 10 in the manner previously described. Theintensity level of the reflected laser beam (as detected by thephotodetector 162) is then compared to the predetermined light intensityrange in order to determine if the confocal optical system 158 isfocused on the front side 38 of the wafer 10. Thereafter, the positionof the objective lens 158 may again be adjusted if the confocal opticalsystem 152 is not focused on the front side of the semiconductor wafer10.

It should be appreciated that such closed-loop control of the positionof the objective lens 58 continues throughout operation of the polishingsystem 30. In particular, as the semiconductor wafer 10 continues to bepolished, the optics controller 164 continuously adjusts the position ofthe objective lens 158 so as to maintain focus on the front side 38 ofthe wafer 10. During such a time, the movement of the objective lens 58may be monitored in order to determine if the semiconductor wafer 10 hasbeen polished to the desired planarity level. In particular, theintensity level of laser beams reflected from the front side 38 of thesemiconductor wafer 10 changes as the wafer 10 is further polished bythe polishing system 30. More specifically, when the front side 38 ofthe semiconductor wafer 10 has been polished down to the desiredplanarity level, incident laser beams impinged thereon are reflecteddirectly back into the confocal optical system 152. It should beappreciated that such directly reflected laser beams have intensitylevels associated therewith which are generally within the predeterminedlight intensity range.

In contrast, when the front side 38 of the semiconductor wafer 10 hasnot yet been polished down to the desired planarity level and thereforepossesses an uneven or varying surface topography, such as shown in FIG.1C, incident laser beams impinging thereon are scattered or otherwisereflected in numerous different directions thereby preventing detectionthereof with the photodetector 162. As described above, if the opticscontroller 164 determines that the confocal optical system 152 is notfocused on the front side 38 of the semiconductor wafer 10 (i.e. thereflected laser beam is either not detected at all by the photodetector162 or the intensity level of the detected light is not within thepredetermined light intensity range), the optics controller 164 causesthe position of the objective lens 158 to be changed. Hence, during agiven period of time, the objective lens 158 is repositioned a fewernumber of times when the confocal optical system 152 is impinging laserbeams on a relatively planar semiconductor wafer 10 in contrast to awafer 10 which possesses an uneven or varying surface topography. Thisis true since the intensity level of laser beams reflected from a planarfront side 38 of the wafer 10 will generally be within the predeterminedlight intensity range thereby eliminating the need to reposition of theobjective lens 158. Conversely, during a similar period of time, theobjective lens 158 is repositioned a large number of times when theconfocal optical system 152 is impinging laser beams on a semiconductorwafer 10 which possesses an uneven or varying surface topographyrelative to a planar wafer 10. This is true since the intensity level ofthe laser beams reflected from an uneven front side 38 of the wafer 10will generally not be within the predetermined light intensity range (ifdetected at all) thereby necessitating repositioning of the objectivelens 158 a large number of times within the given period of time.

As can be seen from the above-discussion, the objective lens 158 has arate-of-movement value which is dependent on the topography of the frontside 38 of the semiconductor wafer 10. What is meant herein by the term“rate-of-movement value” is the number of occurrences during a giventime period in which the objective lens 158 is repositioned or otherwisephysically moved in order to focus the confocal optical system 152 onthe front side 38 of the semiconductor wafer 10. Hence, therate-of-movement value of the objective lens 158 decreases as the frontside 38 of the semiconductor wafer 10 becomes more planar. Inparticular, the rate-of-movement value of the objective lens 158 will berelatively large when a given semiconductor wafer 10 is initiallypolished since the wafer 10 possesses a relatively uneven or varyingtopography, such as shown in FIG. 1C, during initial polishing thereof.However, as the semiconductor wafer 10 is further polished so as toapproach the desired planarity level, the rate-of-movement value of theobjective lens 158 will decrease.

In order to determine when the semiconductor wafer 10 has been polishedto the desired planarity level, a movement threshold value may beestablished. The movement threshold value is indicative of therate-of-movement value of the objective lens 158 when the confocaloptical system 152 is focused on a semiconductor wafer 10 which has beenpolished to the desired planarity level (i.e. when the planar surface 28has been produced). Hence, during polishing of a given semiconductorwafer 10, if the rate-of-movement value of the objective lens 158 has apredetermined relationship with the movement threshold value, the opticscontroller 164 determines that the wafer 10 has been polished to thedesired planarity level. More specifically, during polishing of thegiven semiconductor wafer 10, if the rate-of-movement value of theobjective lens 158 is equal to or less than the movement thresholdvalue, the optics controller 164 generates a wafer-planarized controlsignal which indicates that the wafer 10 has been polished to thedesired planarity level (i.e. when the planar surface 28 has beenproduced).

Referring now to FIG. 5, there is shown a second embodiment of aconfocal optical system 252 which incorporates the features of thepresent invention therein. The confocal optical system 252 is somewhatsimilar to the confocal optical system 152. Thus, the same referencenumerals are used in FIG. 5 to designate common components which werepreviously discussed in regard to FIG. 4.

The confocal optical system 252 does not include a lens positioningdevice (i.e. the lens positioning device 160 of FIG. 4). Hence, theobjective lens 158 is held stationary during operation of the confocaloptical system 252 thereby preventing monitoring of the rate-of-movementvalue associated with the objective lens 158 during polishing of thesemiconductor wafer 10. Therefore, the optics controller 164 monitorsthe number of occurrences during a given time period in which theconfocal optical system 252 is focused on the front side 38 of thesemiconductor wafer 10 in order to determine when the wafer 10 has beenpolished to the desired planarity level (i.e. when the planar surface 28has been produced). In particular, during polishing of the semiconductorwafer 10, the confocal optical system 252 generates incident laser beamswhich are impinged upon the front side 38 of the semiconductor wafer 10so as to produce reflected laser beams which are reflected from thefront side 38 of the wafer 10. The photodetector 162 detects theintensity level of the reflected laser beams (if such beams arereflected back to the confocal optical system 252). The opticscontroller 164 then determines if the intensity level of the reflectedlaser beam is within the predetermined light intensity range. Asdescribed above, the predetermined light intensity range is generallyindicative of a maximum intensity level associated with laser beamsreflected from a planar surface of the front side 38 semiconductor wafer10 which is located at the focal plane of the confocal optical system252.

Hence, during polishing of the semiconductor wafer 10, if the opticscontroller 164 determines that during a predetermined period of time,the confocal optical system 252 is continuously focused on the frontside 38 of the wafer 10, the optics controller 164 concludes that thewafer 10 has been polished to the desired planarity level (i.e. theplanar surface 28 has been produced). More specifically, if during apredetermined period of time, the optics controller 164 determines thatthe intensity level of each of the laser beams reflected from the frontside 38 of the wafer 10 is within the predetermined light intensityrange, the optics controller 164 generates a wafer-planarized controlsignal which indicates that the wafer 10 has been polished to thedesired planarity level (i.e. the planar surface 28 has been produced).

However, if during the predetermined period of time, the opticscontroller 164 determines that the intensity level of a number of thelaser beams reflected from the front side 38 of the wafer 10 is notwithin the predetermined light intensity range, the optics controller164 does not generate a wafer-planarized control signal. As shall bediscussed below in more detail, absence of the wafer-planarized controlsignal causes the polishing system 30 to continue polishing thesemiconductor wafer 10.

Referring back to FIG. 2, the polishing system 30 also includes apolishing controller 82 for controlling the polishing system 30 in orderto effectuate the desired polishing results for the semiconductor wafer10. In particular, the polishing controller 82 is electrically coupledto the displacement mechanism 60 via a signal line 84 to monitor andcontrollably adjust the polishing path of the semiconductor wafer 10 andthe speed at which the semiconductor wafer 10 is moved across the platenassembly 42.

Moreover, the polishing controller 82 is electrically coupled to theplaten motor 40 via a signal line 86 in order to monitor the outputspeed of the platen motor 40 and hence the rotational velocity of theplaten assembly 42. The polishing controller 82 adjusts the output speedof the platen motor 40 and hence the rotational velocity of the platenassembly 42 as required by predetermined operating parameters.

The polishing controller 82 is electrically coupled to the slurry flowcontrol mechanism 76 via a signal line 88 in order to monitor the flowrate of the chemical slurry onto the polishing pad 50 of the platenassembly 42. The polishing controller 82 adjusts the flow rate of thechemical slurry onto the polishing pad 50 of the platen assembly 42 asrequired by predetermined operating parameters.

The polishing controller 82 is further electrically coupled to the wafercarrier motor 58 via a signal line 90 in order to monitor the outputspeed of the wafer carrier motor 58 and hence the rotational velocity ofthe wafer carrier 54. The polishing controller 82 adjusts the outputspeed of the wafer carrier motor 58 and hence the rotational velocity ofthe wafer carrier 54 as required by predetermined operating parameters.It should be appreciated that upon polishing the semiconductor wafer 10to the desired planarity level, the wafer carrier motor 58 may be idledso as to cease polishing of the semiconductor wafer 10. What is meantherein by the term “idled” is that power is cutoff to the wafer carriermotor 58 thereby preventing the wafer carrier motor 58 from driving orotherwise contributing mechanical work to the rotation of the wafercarrier 54.

The polishing controller 82 is also electrically coupled to the confocaloptical system 152, 252 via a signal line 92 in order to determine whenthe semiconductor wafer 10 has been polished to the desired planaritylevel (i.e. the planar surface 28 has been produced). In particularregard to when the endpoint detection system 150 is embodied to includethe confocal optical system 152 (as opposed to the confocal opticalsystem 252), the polishing controller 82 is configured to determine therate-of-movement value of the objective lens 158 and thereafterdetermine if the rate-of-movement value of the objective lens 158 has apredetermined relationship (e.g. is less than or equal to) the movementthreshold value. More specifically, the polishing controller 82 isconfigured to scan or otherwise read the signal line 92 in order todetermine if the confocal optical system 152 has generated awafer-planarized control signal which, as described above, is indicativeof the rate-of-movement value of the objective lens 158 having apredetermined relationship (e.g. is less than or equal to) the movementthreshold value.

In particular regard to when the endpoint detection system 150 includesthe confocal optical system 252, the polishing controller 82 isconfigured to analyze laser beams reflected from the front side 38 ofthe semiconductor wafer 10 in order to determine if the confocal opticalsystem 252 is continuously focused on the front side 38 of the wafer 10for a predetermined period of time. More specifically, the polishingcontroller 82 is configured to scan or otherwise read the signal line 92in order to determine if the confocal optical system 252 has generated awafer-planarized control signal which, as described above, is indicativeof the confocal optical system 252 being continuously focused on thefront side 38 of the wafer 10 for a predetermined period of time.

In operation, the polishing system 30 polishes the semiconductor wafer10 in order to planarize the front side 38 thereof. In particular, thepolishing system 30 removes material from the front side 38 of thesemiconductor wafer 10 until the wafer 10 is polished to the desiredplanarity level (i.e. the planar surface 28 has been formed). Morespecifically, the wafer carrier 54 engages the back side 70 of thesemiconductor wafer 10 and presses the front side 38 of thesemiconductor wafer 10 against the polishing pad 50. The polishingcontroller 82 then causes the platen motor 40 to rotate the platenassembly 42 and the wafer carrier motor 58 to rotate the wafer carrier54. The polishing controller 82 may also begin to control thedisplacement mechanism 60 so as to move the wafer carrier 54 along apredetermined polishing path. The slurry flow control mechanism 76 isalso controlled by the polishing controller 82 in order to applychemical slurry to the polishing pad 50 at a predetermined flow rate.The resulting complex movement of the wafer carrier 54 relative to thepolishing pad 50, the downward force being applied to the semiconductorwafer 10 in the general direction of arrow 62 of FIG. 2, and thechemical slurry all cooperate to selectively remove material from thefront side 38 of the semiconductor wafer 10.

In addition, the polishing controller 82 communicates with the confocaloptical system 152, 252 in order to determine if the semiconductor wafer10 has been polished the desired planarity level. In particular, theconfocal optical system 152, 252 generates incident laser beams whichare impinged on the front side 38 of the wafer 10 during polishingthereof thereby forming reflected laser beams which are reflected fromthe front side 38 of the wafer 10. The reflected laser beams are thenanalyzed (if detected at all by the photodetector 162) by the confocaloptical system 152, 252.

In particular regard to the confocal optical system 152, the intensitylevel of the reflected laser beams causes repositioning of the objectivelens 158 in the manner discussed above. If the rate-of-movement value ofthe objective lens 158 is less than or equal to a movement threshold,the confocal optical system 152 generates a wafer-planarized controlsignal which is sent to the polishing controller 82. In response toreceiving the wafer-planarized control signal, the polishing controller82 ceases polishing the semiconductor wafer 10.

In regard to the confocal optical system 252, the intensity level of thereflected laser beams is utilized to determine if the confocal opticalsystem 252 is continuously focused on the front side 38 of thesemiconductor wafer 10. In particular, if during a predetermined periodof time, the confocal optical system 252 determines that the intensitylevel of each of the laser beams reflected from the front side 38 of thewafer 10 is within the predetermined light intensity range, the confocaloptical system 252 generates a wafer-planarized control signal which issent to the polishing controller 82. In response to receiving thewafer-planarized control signal, the polishing controller 82 ceasespolishing the semiconductor wafer 10.

A polishing procedure 300 utilized by the polishing system 30 to polishthe semiconductor wafer 10 according to the present invention is shownin FIG. 6. The polishing procedure 300 begins with step 302 in which thepolishing controller 82 causes the polishing system 30 to beginpolishing the front side 38 of the semiconductor wafer 10 in order toremove material therefrom. In particular, the polishing controller 82actuates the platen motor 40 in order to cause the platen assembly 42 tobe rotated. Thereafter, the polishing controller 82 actuates the wafercarrier motor 58 thereby causing the wafer carrier 54 and hence thesemiconductor wafer 10 to be rotated so as to polish the front side 38of the semiconductor wafer 10 against the rotating platen assembly 42.The polishing controller 82 also actuates the displacement mechanism 60in order to cause the displacement mechanism 60 to selectively move thewafer carrier 54 and hence the wafer 10 along a predetermined polishingpath. Moreover, the polishing controller 82 causes the chemical slurrysupply system 72 to apply chemical slurry to the polishing pad 50 of theplaten assembly 42 in order to facilitate the removal of material fromthe front side 38 of the semiconductor wafer 10. The procedure 300 thenadvances to step 304.

In step 304, the polishing controller 82 communicates with the confocaloptical system 152, 252 in order to detect the planarity level of thesemiconductor wafer 10 during polishing thereof. In particular, theoptics controller 164 generates an output signal on the signal line 176(see FIGS. 4 and 5) thereby causing the laser source 154 to generateincident laser beams which are impinged upon the front side 38 of thesemiconductor wafer 10. Reflected laser beams which are reflected fromthe front side 38 of the semiconductor wafer 10 are directed through theobjective lens 158 (if reflected in a direction toward the confocaloptical system 152, 252), the beam splitter 156, and thereafter detectedby the photodetector 162. The reflected laser beams are then analyzed bythe confocal optical system 152, 252.

In particular regard to the confocal optical system 152, the intensitylevel of the reflected laser beams causes repositioning of the objectivelens 158 in the manner discussed above in regard to FIG. 4. If therate-of-movement value of the objective lens 158 is less than or equalto a movement threshold value, the confocal optical system 152 generatesa wafer-planarized control signal which is sent to the polishingcontroller 82. If the rate-of-movement value of the objective lens 158is greater than the movement threshold value, a wafer-planarized controlsignal is not generated by the confocal optical system 152.

In regard to the confocal optical system 252, the intensity level of thereflected laser beams is utilized to determine if the confocal opticalsystem 252 is continuously focused on the front side 38 of thesemiconductor wafer 10. In particular, if during a predetermined periodof time, the confocal optical system 252 determines that the intensitylevel of each of the laser beams reflected from the front side 38 of thewafer 10 is within the predetermined light intensity range, the confocaloptical system 252 generates a wafer-planarized control signal which issent to the polishing controller 82. However, if during thepredetermined period of time, the optics controller 164 determines thatthe intensity level of a number of the laser beams reflected from thefront side 38 of the wafer 10 is not within the predetermined lightintensity range, the optics controller 164 does not generate awafer-planarized control signal.

The procedure then advances to step 306 in which the polishingcontroller 82 determines if the front side 38 of the semiconductor wafer10 has been polished to the desired planarity level. In particular, thepolishing controller 82 scans or otherwise reads the signal line 92 inorder to determine if the confocal optical system 152, 252 has generateda wafer-planarized control signal. As described above, presence of awafer-planarized control signal on the signal line 92 indicates that thesemiconductor wafer 10 has been polished to the desired planarity level(i.e. the planar surface 28 has been formed). Hence, in step 306, if thepolishing controller 82 does not detect presence of a wafer-planarizedcontrol signal on the signal line 92, the polishing controller 82concludes that the front side 38 of the semiconductor wafer 10 has notbeen polished to the desired planarity level, and the procedure 300advances to step 308. However, if the polishing controller 82 doesdetect generation of a wafer-planarized control signal on the signalline 92, the polishing controller 82 concludes that the front side 38 ofthe semiconductor wafer 10 has been polished to the desired planaritylevel, and the procedure 300 advances to step 310.

In step 308, the polishing controller 82 communicates with the platenmotor 40, the wafer carrier motor 58, the displacement mechanism 60, andthe slurry flow control 76 in order to continue polishing thesemiconductor wafer 10 in the manner previously discussed. The procedure300 then loops back to step 304 in order to further monitor theplanarization of the semiconductor wafer 10 during subsequent polishingthereof.

Returning now to step 306, if the front side 38 of the semiconductorwafer 10 has been polished to the desired planarity level, the procedure300 advances to step 310. In step 310, the polishing controller 82ceases polishing of the semiconductor wafer 10. In particular, thepolishing controller 82 communicates with the platen motor 40, the wafercarrier motor 58, the displacement mechanism 60, and the slurry flowcontrol 76 in order to cease polishing of the semiconductor wafer 10.However, it should be appreciated that the polishing controller 82 mayallow the polishing system 30 to continue polishing the semiconductorwafer 10 for a short, predetermined amount of time in order to furtherremove material from the semiconductor wafer 10. This further removal ofmaterial or overpolishing may be desirable after certain steps of afabrication process. The procedure 300 then ends thereby placing thepolishing system 30 in an idle state until actuated to polish asubsequent semiconductor wafer.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and description isto be considered as exemplary and not restrictive in character, it beingunderstood that only preferred embodiments have been shown and describedand that all changes and modifications that come within the spirit ofthe invention are desired to be protected.

For example, although the confocal optical system 152 is hereindescribed as monitoring the rate-of-movement value of the objective lens158, and thereby producing numerous advantages in the present invention,certain of such advantages may be achieved by monitoring other values inorder to determine when the semiconductor wafer 10 has been polisheddown to the desired planarity level. For example, the confocal opticalsystem 152 may be configured to monitor a range-of-movement valueassociated with the objective lens 158. What is meant herein by the term“range-of-movement value” is the physical distance in which theobjective lens 158 is repositioned or otherwise physically moved duringa given time period in order to focus the confocal optical system 152 onthe front side 38 of the semiconductor wafer 10. It should beappreciated that during a given period of time, the objective lens 158is repositioned a fewer number of times (and therefore traverses arelatively small distance) when the confocal optical system 152 isimpinging laser beams on a relatively planar semiconductor wafer 10 incontrast to a wafer 10 which possesses an uneven or varying surfacetopography. This is true since the intensity level of laser beamsreflected from a planar front side 38 of the wafer 10 will generally bewithin the predetermined light intensity range thereby eliminating theneed to reposition of the objective lens 158. Conversely, during asimilar period of time, the objective lens 158 is repositioned a largenumber of times (and therefore traverses a relatively large distance)when the confocal optical system 152 is impinging laser beams on asemiconductor wafer 10 which possesses an uneven or varying surfacetopography relative to a planar wafer 10. This is true since theintensity level of the laser beams reflected from an uneven front side38 of the wafer 10 will generally not be within the predetermined lightintensity range (if detected at all) thereby necessitating repositioningof the objective lens 158 a large number of times within the givenperiod of time.

For further example, it should be appreciated that although thepolishing system 30 and the confocal optical system 152, 252 are hereindescribed as having separate controllers (i.e. the polishing controller82 and the optics controller 164, respectively), it should beappreciated that a single controller may be provided to control both thepolishing system 30 and the confocal optical system 152, 252.

What is claimed is:
 1. A method of planarizing a first side of asemiconductor wafer with a polishing system, comprising the steps of:polishing said first side of said wafer in order to remove material fromsaid wafer; moving a lens of a confocal optical system between a numberof lens positions so as to maintain focus on said first side of saidwafer during said polishing step; determining a rate-of-movement valuebased on movement of said lens during said moving step; and stoppingsaid polishing step if said rate-of-movement value has a predeterminedrelationship with a movement threshold value.
 2. The method of claim 1,wherein said stopping step includes the step of stopping said polishingstep if said rate-of-movement value is less than said movement thresholdvalue.
 3. The method of claim 1, wherein: said polishing step includesthe step of rotating said wafer with a wafer motor, and said stoppingstep includes the step of idling said wafer motor if saidrate-of-movement value has said predetermined relationship with saidmovement threshold.
 4. The method of claim 1, wherein: said polishingstep includes the step of rotating a wafer carrier so as to urge saidwafer into contact with a polishing platen, and said confocal opticalsystem is positioned such that an incident light beam emitting therefromis directed through an opening defined in said polishing platen so as toimpinge upon said first side of said wafer.
 5. The method of claim 1,wherein said moving step includes the step of moving an objective lensof said confocal optical system between said number of lens positions soas to maintain focus on said first side of said wafer during saidpolishing step.
 6. A method of planarizing a first side of asemiconductor wafer, comprising the steps of: polishing said first sideof said wafer in order to remove material from said wafer; transmittinga first incident light beam from a confocal optical system during afirst time period, wherein said first incident light beam impinges onsaid first side of said wafer during said polishing step so as to form afirst reflected light beam which is reflected from said first side ofsaid wafer; analyzing said first reflected light beam so as to determineif said confocal optical system is focused on said first side of saidwafer during said first time period; transmitting a second incidentlight beam from said confocal optical system during a second timeperiod, wherein said second incident light beam impinges on said firstside of said wafer during said polishing step so as to form a secondreflected light beam which is reflected from said first side of saidwafer; analyzing said second reflected light beam so as to determine ifsaid confocal optical system is focused on said first side of said waferduring said second time period; and stopping said polishing step if saidconfocal optical system is focused on said first side of said waferduring both said first time period and said second time period.
 7. Themethod of claim 6, wherein: said polishing step includes the step ofrotating said wafer with a wafer motor, and said stopping step includesthe step of idling said wafer motor if said confocal optical system isfocused on said first side of said wafer during both said first timeperiod and said second time period.
 8. The method of claim 6, wherein:said polishing step includes the step of rotating a wafer carrier so asto urge said wafer into contact with a polishing platen, and saidconfocal optical system is positioned such that both said first and saidsecond incident light beams are directed through an opening defined insaid polishing platen.
 9. The method of claim 8, wherein: said firstincident light beam includes a first incident laser beam, said secondincident light beam includes a second incident laser beam, said step oftransmitting said first incident light beam includes the step ofgenerating said first incident laser beam with a laser source unit, saidstep of transmitting said second incident light beam includes the stepof generating said second incident laser beam with said laser sourceunit, and said laser source unit is positioned such that both said firstand second incident laser beams are directed through said openingdefined in said polishing platen so as to impinge on said first side ofsaid wafer during said polishing step.
 10. An apparatus for polishing afirst side of a semiconductor wafer, comprising: a polishing systemwhich operates to polish said wafer, said polishing system having (i) apolishing platen which includes a polishing surface, and (ii) a wafercarrier which is configured to (a) engage said wafer by a second side ofsaid wafer, and (b) apply pressure to said wafer in order to press saidwafer against said polishing surface of said polishing platen; aconfocal optical system having a movable objective lens, said confocaloptical system being configured to move said objective lens between anumber of lens positions so as to maintain focus on said first side ofsaid wafer during polishing of said wafer; and a controller electricallycoupled to said confocal optical system, wherein said controller isconfigured to (i) determine a rate-of-movement value based on movementof said objective lens during polishing of said wafer, and (ii)terminate operation of said polishing system so as to cease polishing ofsaid wafer in response to determination that said rate-of-movement valuehas a predetermined relationship with a movement threshold value. 11.The apparatus of claim 10, further comprising a wafer motor, wherein:said wafer motor is operable in (i) a polishing mode of operation inwhich said wafer motor rotates said wafer carrier, and (ii) an idle modeof operation in which said wafer motor is idle, and said wafer motor ispositioned in said idle mode of operation if said rate-of-movement valuehas said predetermined relationship with said movement threshold. 12.The apparatus of claim 10, wherein: said polishing platen has an openingdefined therein, and said confocal optical system is positioned suchthat an incident light beam emitting therefrom is directed through saidopening defined in said polishing platen so as to be impinged upon saidfirst side of said wafer.
 13. The apparatus of claim 12, wherein: saidconfocal optical system includes a confocal laser system, said incidentlight beam includes an incident laser beam generated by said confocallaser system, and said confocal laser system is positioned such thatsaid incident laser beam is directed through said opening defined insaid polishing platen so as to impinge said incident laser beam on saidfirst side of said wafer.
 14. The apparatus of claim 10, wherein saidcontroller is further configured to terminate operation of saidpolishing system so as to cease polishing of said wafer in response todetermination that said rate-of-movement value is less than saidmovement threshold value.
 15. An apparatus for polishing a first side ofa semiconductor wafer, comprising: a polishing system which operates topolish said wafer, said polishing system having (i) a polishing platenwhich includes a polishing surface, and (ii) a wafer carrier which isconfigured to (a) engage said wafer by a second side of said wafer, and(b) apply pressure to said wafer in order to press said wafer againstsaid polishing surface of said polishing platen; a confocal opticalsystem positioned such that (i) a first incident light beam transmittedby said confocal optical system is impinged upon said first side of saidwafer during a first period of time so as to form a first reflectedlight beam which is reflected from said first side of said wafer, (ii) asecond incident light beam transmitted by said confocal optical systemis impinged upon said first side of said wafer during a second period oftime so as to form a second reflected light beam which is reflected fromsaid first side of said wafer, and (iii) said first and second reflectedlight beams are received with said confocal optical system; and acontroller electrically coupled to said confocal optical system, whereinsaid controller is configured to (i) analyze said first reflected lightbeam so as to determine if said confocal optical system is focused onsaid first side of said wafer during said first time period, (ii)analyze said second reflected light beam so as to determine if saidconfocal optical system is focused on said first side of said waferduring said second time period, and (iii) terminate operation of saidpolishing system so as to cease polishing of said wafer in response todetermination that said confocal optical system is focused on said firstside of said wafer during both said first time period and said secondtime period.
 16. The apparatus of claim 15, further comprising a wafermotor, wherein: said wafer motor is operable in (i) a polishing mode ofoperation in which said wafer motor rotates said wafer carrier, and (ii)an idle mode of operation in which said wafer motor is idle, and saidwafer motor is positioned in said idle mode of operation in response todetermination that said confocal optical system is focused on said firstside of said wafer during both said first time period and said secondtime period.
 17. The apparatus of claim 15, wherein: said polishingplaten has an opening defined therein, and said confocal optical systemis positioned such that (i) said first and second incident light beamsare directed through said opening defined in said polishing platen so asto be impinged upon said first side of said wafer, and (ii) said firstand second reflected light beams are directed through said opening so asto be received with said confocal optical system.
 18. The apparatus ofclaim 17, wherein: said confocal optical system includes a confocallaser system, said first and second incident light beams include firstand second incident laser beams generated by said confocal laser system,said first and second reflected light beams include first and secondreflected laser beams reflected from said first side of said wafer, andsaid confocal laser system is positioned such that (i) said first andsecond incident laser beams are directed through said opening defined insaid polishing platen so as to impinge said first and second incidentlaser beams on said first side of said wafer, and (ii) said first andsecond reflected laser beams are received by said confocal laser system.